The present invention relates to an image recording apparatus, and more particularly to a high precision image recording apparatus suitable for repetitively recording a same image on a plurality of recording sheets with high precision.
In a conventional recording apparatus of this type, for example, as disclosed in Japanese Laid-open Patent Publication No. JP-A-59-69765, the surface of a photosensitive drum driven via a gear train, timing belt or chain at a constant speed with a motor rotating at high speed is uniformly charged with a copying exposure apparatus, laser exposure apparatus or LED exposure apparatus to thereby form a latent image thereon. The latent image is transformed into a toner image with a developing unit and transferred on a recording sheet to obtain a copy or printed matter.
In one of conventional recording apparatus, the circumferential length of a transfer drum which supports and endlessly moves a transfer sheet is set at an integer-fold length of that of the photosensitive drum (JP-A-61-83557).
In another of conventional recording apparatus, the rotating speed of the photosensitive drum is changed during the time while a new recording sheet is fed to a print position so that writing a image always starts at a fixed position on the photosensitive drum to thereby minimize magnification/reduction of an image print sample (JP-A-63-78163).
Further in another of conventional recording apparatus, the paper feed timing is regulated so that writing an image always starts at a fixed position on the photosensitive drum to thereby minimize magnification/reduction of an image print sample (JP-A-63-78164).
The above-described three Japanese Patent Laid-open Publications (JP-A-61-83557, JP-A-63-78163, and JP-A-63-78164) do not refer to eccentricity of the drive unit (gear, timing belt or the like) for the photosensitive drum. If the rotational speed of the photosensitive drum varies independently during each revolution cycle thereof and if the write start positions only are merely set in positional alignment, it is not possible to minimize magnification/reduction of an image print sample. Particularly in the last two Japanese Patent Laid-open Publications (JP-A-63-78163 and JP-A-63-78164), if the paper feed roller is eccentric relative to its axis, the paper feed speed at the transfer position varies and results in a certain magnification/reduction of an image print sample.
As mentioned above, in the conventional apparatus, the rotation fluctuation of a photosensitive member and the recording sheet feed speed fluctuation have not been sufficiently considered. There arises therefore the problem that the pitch fluctuation of an image in the sub-scan direction (recording sheet feed direction) becomes large. The reasons for this will be described in the following:
First, the driving source or motor rotating at high speed causes the photosensitive member to rotate at a reduced speed by means of a multiple gear train. Therefore, the rotation fluctuation of the photosensitive member depends upon the accuracy of gear shape, gear circularity, errors of distance between gear shafts, gear meshing errors, and the like.
Second, an electrophotographic processing unit including a cleaning unit and developing unit in contact with the surface of the photosensitive member exerts a load to the latter. The cleaning unit in particular changes its contact state with the photosensitive member in accordance with the amount of toner. Such load change is one of the reasons of the rotation fluctuation of the photosensitive member.
Third, rollers for transporting a recording sheet are driven via a gear train, timing belt, chain or the like by a drive system for the photosensitive member. In addition, a clutch and the like are used for controlling the recording sheet feed timing. These load changes are applied to the photosensitive member via the drive system to thereby cause the rotation fluctuation of the photosensitive member.
If the rotation fluctuation of the photosensitive member is present because of these reasons, there occurs a fluctuation in print pitch between images exposed with an optical device, the print pitch fluctuation being in correspondence with the rotation fluctuation of the photosensitive member.
In addition, there is no consideration of precision regarding the gear train, timing belt, chain or the like for driving the recording sheet transport rollers. Even if such consideration is made, there is a precision limit in practical manufacture, thereby generating a fluctuation of recording sheet transport speed. This transport speed fluctuation results in a transfer fluctuation and print pitch fluctuation.
The print pitch fluctuation results from the above reasons. The print pitch fluctuation varies for each of recording sheets so that if a same image is printed on a plurality of recording sheets, there is a shift in position between printed images.
It is an object of the present invention to reduce the print pitch fluctuation, make the reduced print pitch fluctuation to occur at same positions on each recording sheet to thereby avoid the generation of positional shift between printed images.